1. Field of the Invention
The invention generally relates to electronics. In particular, the invention relates to drivers for low voltage differential signaling.
2. Description of the Related Art
Low-Voltage Differential Signaling (LVDS) is a popular technology for high performance data transmission applications. LVDS is growing in popularity for differential data transmission because it features relatively high speed and relatively low-power. Other benefits of LVDS include: low-voltage power supply compatibility; relatively low EMI generation; relatively high noise rejection; robust transmission signals; and an ability to be integrated into system level ICs. LVDS technology allows products to address high data rates ranging from, for example, hundreds of Mbps to greater than 2 Gbps.
LVDS uses differential data transmission to reduce susceptibility to common-mode noise. This permits the swing levels to be lower, which significantly reduces power dissipation. However, maintaining swing levels over wide parametric variations can be challenging. The control of swing level can be an important aspect to an LVDS system, as the worst case swing level defines the power dissipation of the LVDS system.
As illustrated in FIG. 1, a transmission medium 102 should be terminated 104 to its characteristic differential impedance to complete the current loop and to terminate the high-speed signals. Termination should be applied whether the LVDS transmission medium includes a cable or controlled impedance traces on a printed circuit board. Typically, to prevent reflections, LVDS also uses a near-end terminating resistor 106 that is matched to the actual cable or PCB trace's differential impedance as close as possible to the driver output. The near-end terminating resistor 106 however results in additional power loss in those resistors.
An LVDS driver is desirably hot-pluggable. A hot-pluggable LVDS driver should not consume an inordinate amount of current through the output pins when switched off. Fault detection of the output pins of the hot-pluggable LVDS driver should be used. Fault detection circuits detect and prevent excessive current flow arising out of, for example, accidental short circuits to power or ground.
FIG. 1 illustrates a conventional approach to an LVDS driver implemented with a constant current driver (also called current mode driver) with explicit near-end passive termination 106. The conventional current mode driver illustrated in FIG. 1 is only about 50% efficient. This is because the current gets divided between the far end termination 104 and the explicit near end termination 106.
FIG. 1 above shows an ideal current driver; however realizable current sources have wide parametric variations, resulting in significant swing variations over the line.
Such schemes would typically require a CMFB (Common Mode Feedback Control) circuit to control the common mode level of the driver. Discussed below are various implementations of the LVDS drivers which are variations on the classical theme discussed above.
For example, U.S. Pat. No. 6,111,431 to Estrada illustrates a current mode driver. However, the Vds (Drain-Source Voltage) of the current sources are pinned to specific values through a feedback system to control the swing level. Such a scheme uses a near-end termination, resulting in power and speed limitations. U.S. Pat. No. 6,600,346 to Macaluso has similar limitations.
U.S. Pat. No. 6,731,135 to Brunolli illustrates another variation of an LVDS driver. In Brunolli, the common mode voltage is provided using a feedback system to control the output swing levels. See, for example, FIG. 3 of Brunolli.
U.S. Pat. No. 6,867,618 to Li, et al., has a low-output impedance structure as compared to high impedance constant current sources. See, for example, FIG. 3 of Li. However, there is no control of the swing levels.
U.S. Pat. No. 7,012,450 to Oner, et al, illustrates a self-termination driver with feedback to control the common mode voltage of the driver. It can be relatively intrusive to perform feedback control of common mode voltage because the feedback path is then embedded in the signal path (differential or common mode). In addition, the feedback control controls only the common mode voltage and not the differential swing. Thus, the differential signal swing still varies over process, voltage, and temperature (PVT) corners.
FIG. 2 of U.S. Pat. No. 6,411,146 to Kuo illustrates tracking a substrate voltage level on a shared bus. Kuo's technique avoids parasitic diode leakage. See also U.S. Pat. No. 7,068,077 to Reinschmidt for relevant art.